Particulate combustion catalyst, particulate filter and exhaust gas purifying apparatus

ABSTRACT

Provided is a particulate combustion catalyst including a carrier formed of Zr oxide, a carrier formed of a Zr—Ce composite oxide, or a carrier formed of a composite oxide containing Zr, Ce, and at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr; an oxide of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr, the metal oxide being supported on the carrier in an amount, as reduced to metal, of 0.5 to 30 mass % with respect to the carrier; and metallic Ag or Ag oxide, which serves as a catalyst component and is supported on the carrier. Also provided are a particulate filter coated with the catalyst; and an exhaust gas cleaning apparatus including a particulate filter coated with the catalyst.

TECHNICAL FIELD

The present invention relates to a particulate combustion catalyst, to aparticulate filter, and to an exhaust gas cleaning apparatus. Moreparticularly, the present invention relates to a particulate combustioncatalyst which realizes removal (through oxidation) of particulatematter discharged from a diesel internal combustion engine; to aparticulate filter coated with the particulate combustion catalyst; andto an exhaust gas cleaning apparatus including the particulate filtercoated with the particulate combustion catalyst.

BACKGROUND ART

Exhaust gas discharged from diesel engines contains nitrogen oxides(NO_(x)) and particulate matter, and release of such substances into theatmosphere without any treatment is a main cause of air pollution.Therefore, demand has arisen for strict regulation for such substances.There has been proposed, as effective means for removing particulatematter, a flow-through oxidation catalyst for combustion of solubleorganic fractions (SOFs), and a diesel exhaust gas trapping systememploying a diesel particulate filter for trapping soot. However, forregeneration of such a particulate filter, particulate matter trappedtherein must be continuously removed through oxidation.

Hitherto, a variety of continuous regeneration systems have beenproposed, and examples thereof include a system employing a catalystincluding a carrier made of an inorganic oxide (e.g., zirconium oxide,vanadium oxide, or cerium oxide), and an expensive noble metal (e.g.,Pt) supported on the carrier (see, for example, Patent Document 1, 2, or3); and a continuous regeneration method involving NO₂ (see, forexample, Patent Document 4). This continuous regeneration methodrequires provision, upstream of a particulate filter, of an oxidationcatalyst (e.g., Pt) for oxidizing NO into NO₂, and thus involves highcost. In addition, reaction involving NO₂ is affected by the ratio ofNO_(x) to C, and many restrictions are imposed on the employment of thismethod.

Patent Document 1: Japanese Patent Application Laid-Open (kokai) No.H10-047035Patent Document 2: Japanese Patent Application Laid-Open (kokai) No.2003-334443Patent Document 3: Japanese Patent Application Laid-Open (kokai) No.2004-058013

Patent Document 4: Japanese Patent No. 3012249 DISCLOSURE OF THEINVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a particulatecombustion catalyst which exhibits excellent heat resistance, whichrealizes removal of soot through oxidation at low temperature withoutemployment of an expensive noble metal, and which enables oxidationreaction to proceed with the aid of only oxygen and thus realizesremoval of soot through oxidation at low temperature regardless of theNO_(x) concentration of exhaust gas. Another object of the presentinvention is to provide a particulate filter coated with the particulatecombustion catalyst. Yet another object of the present invention is toprovide an exhaust gas cleaning apparatus comprising the particulatefilter coated with the particulate combustion catalyst.

Means for Solving the Problems

In order to achieve the aforementioned objects, the present inventorshave conducted extensive studies, and as a result have found that theobjects can be achieved by employing, as a carrier of a particulatecombustion catalyst, a specific oxide or a composite oxide having aspecific composition; supporting, on the carrier, an oxide of at leastone metal selected from the group consisting of Ba, Ca, Mg, and Sr; andsupporting, on the carrier, metallic Ag or Ag oxide, which serves as acatalyst component. The present invention has been accomplished on thebasis of this finding.

Accordingly, the present invention provides a particulate combustioncatalyst characterized by comprising a carrier formed of zirconiumoxide; an oxide of at least one metal selected from the group consistingof Ba, Ca, Mg, and Sr, the metal oxide being supported on the carrier inan amount, as reduced to metal, of 0.5 to 30 mass % with respect to thecarrier; and metallic Ag or Ag oxide, which serves as a catalystcomponent and is supported on the carrier.

The present invention also provides a particulate combustion catalystcharacterized by comprising a carrier formed of a zirconium-ceriumcomposite oxide; an oxide of at least one metal selected from the groupconsisting of Ba, Ca, Mg, and Sr, the metal oxide being supported on thecarrier in an amount, as reduced to metal, of 0.5 to 30 mass % withrespect to the carrier; and metallic Ag or Ag oxide, which serves as acatalyst component and is supported on the carrier.

The present invention also provides a particulate combustion catalystcharacterized by comprising a carrier formed of a composite oxidecontaining zirconium, cerium, and at least one metal selected from amongNd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr; an oxide of at least onemetal selected from the group consisting of Ba, Ca, Mg, and Sr, themetal oxide being supported on the carrier in an amount, as reduced tometal, of 0.5 to 30 mass % with respect to the carrier; and metallic Agor Ag oxide, which serves as a catalyst component and is supported onthe carrier.

The present invention also provides a particulate filter characterizedby being coated with any of the aforementioned particulate combustioncatalysts. The present invention also provides an exhaust gas cleaningapparatus characterized by comprising a particulate filter coated withany of the aforementioned particulate combustion catalysts.

EFFECTS OF THE INVENTION

The particulate combustion catalyst of the present invention exhibitsexcellent heat resistance. Employment of the particulate combustioncatalyst of the present invention realizes removal of soot throughoxidation at low temperature without use of an expensive noble metal.When the particulate combustion catalyst is employed, since oxidationreaction proceeds with the aid of only oxygen, soot can be removedthrough oxidation at low temperature regardless of the NO_(x)concentration of exhaust gas. Even when a catalyst system including theparticulate combustion catalyst is exposed to a high-temperatureatmosphere for a long period of time, degradation of the system can besuppressed.

BEST MODES FOR CARRYING OUT THE INVENTION

The particulate combustion catalyst of the present invention employs acarrier formed of zirconium oxide, a carrier formed of azirconium-cerium composite oxide, or a carrier formed of a compositeoxide containing zirconium, cerium, and at least one metal selected fromamong Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr. Among catalysts havingthe same composition (except for a carrier), a catalyst including acarrier formed of a zirconium-cerium composite oxide tends to be higherin performance than a catalyst including a carrier formed of zirconiumoxide, and a catalyst including a carrier formed of a composite oxidecontaining zirconium, cerium, and at least one metal selected from amongNd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr tends to be higher inperformance than a catalyst including a carrier formed of azirconium-cerium composite oxide.

When the particulate combustion catalyst of the present inventionemploys, as a carrier, a zirconium-cerium composite oxide, the ceriumoxide content of the composite oxide is preferably 5 to 50 mass %. Whenthe cerium oxide content exceeds 50 mass %, the specific surface area ofthe carrier is reduced at a high temperature (e.g., 700° C. or higher),which may eventually cause thermal degradation of the catalyst. Inaddition, when the cerium oxide content exceeds 50 mass %, an activespecies may fail to sufficiently exert its performance. In contrast,when the cerium oxide content is less than 5 mass %, the carrierexhibits poor heat resistance, which may eventually cause thermaldegradation of the catalyst.

When the particulate combustion catalyst of the present inventionemploys, as a carrier, a composite oxide containing zirconium, cerium,and at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca,Mg, Sn, and Sr, since the carrier contains an oxide of at least onemetal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr, thecarrier exhibits improved thermal stability, and oxidation property atlow temperature is improved. In order to attain such effects, the amountof an oxide of at least one metal selected from among Nd, La, Fe, Y, Pr,Ba, Ca, Mg, Sn, and Sr is preferably 1 mass % or more. However, when theamount of such a metal oxide exceeds 35 mass %, accordingly, therelative amounts of zirconium oxide and cerium oxide are reduced, andcharacteristics of the carrier containing the zirconium-cerium compositeoxide tend to be deteriorated. Therefore, in the composite oxidecontained in the carrier employed, preferably, the amount of an oxide ofat least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg,Sn, and Sr is 1 to 35 mass % (i.e., when two or more metal oxides areemployed, the total amount of the oxides is 1 to 35 mass %), and thecerium oxide content is 5 to 50 mass % (zirconium oxide content:balance).

In the present invention, an oxide of at least one metal selected fromthe group consisting of Ba, Ca, Mg, and Sr must be supported on any ofthe aforementioned carriers. A conventionally known technique (e.g., theimpregnation method or the sol-gel method) may be employed forsupporting such a metal oxide on the carrier. In the present invention,when an oxide of at least one metal selected from the group consistingof Ba, Ca, Mg, and Sr is supported on the carrier, the resultantparticulate combustion catalyst exhibits improved heat resistance. Inorder to attain such effects, the amount (as reduced to metal) of anoxide of at least one metal selected from among Ba, Ca, Mg, and Sr ispreferably 0.5 mass % or more with respect to the carrier. However, whenthe amount (as reduced to metal) of such a metal oxide exceeds 30 mass %with respect to the carrier, accordingly, the relative amounts ofzirconium oxide and cerium oxide are reduced, and characteristics of thecarrier containing the zirconium-cerium composite oxide tend to bedeteriorated. Therefore, preferably, the amount (as recued to metal) ofan oxide of at least one metal selected from among Ba, Ca, Mg, and Sr is0.5 to 30 mass % (i.e., when two or more metal oxides are employed, thetotal amount (as recued to metal) of the oxides is 0.5 to 30 mass %)with respect to the carrier (i.e., 0.5 to 30 parts by mass on the basisof 100 parts by mass of the carrier).

In the present invention, metallic Ag or Ag oxide must be supported, asa catalyst component, on the carrier. A conventionally known technique(e.g., the impregnation method or the sol-gel method) may be employedfor supporting such a catalyst component on the carrier. Metallic Ag orAg oxide, which is employed in the present invention, is less expensivethan, for example, Pt or Pd. In addition, when metallic Ag or Ag oxideis employed in combination with a specific carrier used in the presentinvention, further excellent effects are obtained, as compared with thecase where a Pt or Pd component is employed. In the present invention,preferably, the amount (as reduced to metal) of metallic Ag or Ag oxidesupported on the carrier is 0.1 to 25 mass % on the basis of the totalmass of the carrier and an oxide of at least one metal selected from thegroup consisting of Ba, Ca, Mg, and Sr. When the amount of the catalystcomponent is less than 0.1 mass %, the catalyst component may fail tosufficiently exhibit its catalytic effects, whereas when the amount ofthe catalyst component exceeds 25 mass %, a specific combinationemployed in the present invention may fail to sufficiently exhibit asynergistic effect. Meanwhile, when the amount of the catalyst componentis large, sintering of metal is likely to occur, and the catalystcomponent is not expected to exhibit its catalytic effects.

In consideration that the particulate filter of the present invention isproduced by causing the particulate combustion catalyst of the presentinvention to be held on a base, preferably, the surface of the carrieris provided with a binder component such as SiO₂, TiO₂, ZrO₂, or Al₂O₃.When such a binder component is provided on the surface of the carrier,adhesion between the base and the carrier is enhanced, and the catalystexhibits improved durability and heat resistance.

The particulate filter of the present invention may assume any knownform of particulate filter, but preferably has a three-dimensionalstructure. Specific examples of filters having a three-dimensionalstructure include a wall-through filter, a flow-through honeycombfilter, a wire mesh filter, a ceramic fiber filter, a metallic porousfilter, a particle-charged filter, and a foam filter. Examples of thematerial of the base include ceramic materials such as cordierite andSiC; Fe—Cr—Al alloys; and stainless steel alloys.

The exhaust gas cleaning apparatus of the present invention, whichincludes therein the aforementioned particulate filter of the presentinvention, will be readily appreciated by those skilled in the art.

Next will be described a method for producing the particulate filter ofthe present invention.

Any of the aforementioned types of carriers is mixed with a bindercomponent (e.g., SiO₂ or alumina sol) and water, and the resultantmixture is finely milled by means of a milling apparatus (e.g., a ballmill). A particulate filter (e.g., a wire mesh filter) is coated withthe thus-obtained slurry. In general, the slurry-coated filter is firedat a temperature of about 500° C. to about 700° C. The thus-formedwash-coating layer is impregnated with, for example, a nitrate of atleast one metal selected from the group consisting of Ba, Ca, Mg, andSr, and then drying and firing are carried out. Subsequently, theresultant product is impregnated with, for example, silver nitrateserving as a catalyst component, and then drying and firing are carriedout. Alternatively, the wash-coating layer may be impregnated with, forexample, a nitrate of at least one metal selected from the groupconsisting of Ba, Ca, Mg, and Sr, together with, for example, silvernitrate, followed by drying and firing. The total catalyst coatingamount is preferably 10 to 100 g/L (for a wall-flow particulate filter)or about 50 to about 150 g/L (for a wire mesh particulate filter). Whenthe total catalyst coating amount is excessively small, sufficientperformance fails to be attained, whereas when the total catalystcoating amount is excessively large, back pressure to exhaust gasincreases.

EXAMPLES

The present invention will next be described in detail with reference toExamples and Comparative Examples. In each of the Examples andComparative Examples, a parenthesized numerical value following each ofthe oxides constituting a composite oxide represents the amount (mass %)of the constitutive oxide.

Example 1

Water (30 g) was added to powder of a composite oxide ofCeO₂(22)ZrO₂(72)La₂O₂(2)Nd₂O₃(4) (20 g), and SiO₂ sol (i.e., a bindercomponent) (5 g, as reduced to SiO₂) was added thereto, followed bymixing for two hours, to thereby prepare a slurry. By use of the slurry,a cordierite-made particulate filter (25.4 mm in diameter×60 mm inlength) was coated with the composite oxide. The composite-oxide-coatedfilter was dried at 120° C. for three hours, and then fired in air at500° C. for one hour. The composite-oxide-coated filter was found tohave a composite oxide content of 40 g/L. The composite-oxide-coatedfilter was impregnated with an aqueous magnesium nitrate solution havinga specific concentration and an aqueous silver nitrate solution having aspecific concentration. The resultant product was dried at 120° C. forthree hours, and then finally fired in air at 500° C. for one hour. Thefinally formed filter was found to have an Ag content of 5 g/L and an Mgcontent of 1 g/L. The Ag content as determined on the basis of the totalmass of the aforementioned composite oxide and magnesium oxide was 12mass %, and the Mg content as determined on the basis of the mass of theaforementioned composite oxide was 2.5 mass %.

Example 2

The procedure of Example 1 was repeated, except that the aqueousmagnesium nitrate solution was replaced with an aqueous calcium nitratesolution. The finally formed filter was found to have an Ag content of 5g/L and a Ca content of 1 g/L. The Ag content as determined on the basisof the total mass of the aforementioned composite oxide and calciumoxide was 12.1 mass %, and the Ca content as determined on the basis ofthe mass of the aforementioned composite oxide was 2.5 mass %.

Example 3

The procedure of Example 1 was repeated, except that the aqueousmagnesium nitrate solution was replaced with an aqueous barium nitratesolution. The finally formed filter was found to have an Ag content of 5g/L and a Ba content of 1 g/L. The Ag content as determined on the basisof the total mass of the aforementioned composite oxide and barium oxidewas 12.2 mass %, and the Ba content as determined on the basis of themass of the aforementioned composite oxide was 2.5 mass %.

Example 4

The procedure of Example 1 was repeated, except that the aqueousmagnesium nitrate solution was replaced with an aqueous strontiumnitrate solution. The finally formed filter was found to have an Agcontent of 5 g/L and an Sr content of 1 g/L. The Ag content asdetermined on the basis of the total mass of the aforementionedcomposite oxide and strontium oxide was 12.1 mass %, and the Sr contentas determined on the basis of the mass of the aforementioned compositeoxide was 2.5 mass %.

Example 5

The procedure of Example 1 was repeated, except that the composite oxideof CeO₂(22)ZrO₂(72)La₂O₃(2)Nd₂O₃(4) was replaced with ZrO₂, and theaqueous magnesium nitrate solution was replaced with an aqueous bariumnitrate solution. The finally formed filter was found to have an Agcontent of 5 g/L and a Ba content of 1 g/L. The Ag content as determinedon the basis of the total mass of ZrO₂ and barium oxide was 12.2 mass %,and the Ba content as determined on the basis of the mass of ZrO₂ was2.5 mass %.

Example 6

The procedure of Example 1 was repeated, except that the composite oxideof CeO₂(22)ZrO₂(72)La₂O₃(2)Nd₂O₃(4) was replaced with a composite oxideof CeO₂(30)ZrO₂(70), and the aqueous magnesium nitrate solution wasreplaced with an aqueous barium nitrate solution. The finally formedfilter was found to have an Ag content of 5 g/L and a Ba content of 1g/L. The Ag content as determined on the basis of the total mass of theaforementioned composite oxide and barium oxide was 12.2 mass %, and theBa content as determined on the basis of the mass of the aforementionedcomposite oxide was 2.5 mass %.

Example 7

Water (700 g) was added to powder of a composite oxide ofCeO₂(22)ZrO₂(72)La₂O₃(2)Nd₂O₃(4) (100 g), and the resultant mixture wasmilled by means of a ball mill so as to attain a mean particle size of 1μm or less. Subsequently, ZrO₂ sol (i.e., a binder component) (10 g, asreduced to ZrO₂) was added to the mixture, followed by mixing for twohours, to thereby prepare a slurry. By use of the slurry, acordierite-made particulate filter (25.4 mm in diameter×76.2 mm inlength) was coated with the composite oxide. The composite-oxide-coatedfilter was dried at 120° C. for three hours, and then fired in air at500° C. for one hour. The composite-oxide-coated filter was found tohave a composite oxide content of 40 g/L. The composite-oxide-coatedfilter was impregnated with an aqueous barium nitrate solution having aspecific concentration and an aqueous silver nitrate solution having aspecific concentration. The resultant product was dried at 120° C. forthree hours, and then finally fired in air at 500° C. for one hour. Thefinally formed filter was found to have an Ag content of 2 g/L and a Bacontent of 2 g/L. The Ag content as determined on the basis of the totalmass of the aforementioned composite oxide and barium oxide was 4.7 mass%, and the Ba content as determined on the basis of the mass of theaforementioned composite oxide was 5 mass %.

Comparative Example 1

The procedure of Example 1 was repeated, except that the aqueousmagnesium nitrate solution was not employed. The finally formed filterwas found to have an Ag content of 5 g/L, and the Ag content asdetermined on the basis of the mass of the composite oxide was 12.5 mass%.

Comparative Example 2

The procedure of Example 7 was repeated, except that the aqueous bariumnitrate solution was not employed. The finally formed filter was foundto have an Ag content of 2 g/L, and the Ag content as determined on thebasis of the mass of the composite oxide was 5 mass %.

<Evaluation of Catalyst-Coated Particulate Filter in Terms of SootCombustion by Use of Simulated Exhaust Gas>

The Tig (combustion initiation temperature) of soot corresponding toeach of the catalyst-coated particulate filters produced in Examples 1to 6 and Comparative Example 1 was measured through the followingmethod.

A specific amount of a dispersion prepared by dispersing carbon(Printex-V (toner carbon), product of Degussa) (20 mg) in ethyl alcoholwas added dropwise to each of the catalyst-coated particulate filtersproduced in Examples 1 to 6 and Comparative Example 1 (25.4 mm indiameter×60 mm in length) from above the filter, followed by drying at100° C. for 10 minutes. Thus, carbon (20 mg) was deposited on onecatalyst-coated particulate filter. The carbon-deposited filter wasfixed at a center portion of a quartz-made simulated exhaust gasreaction tube. While a circulation gas having the below-describedcomposition was caused to flow through the quartz reaction tube at thebelow-described flow rate, the temperature of the reaction tube waselevated at the below-described temperature elevation rate by means ofan electric furnace, and CO and CO₂ concentrations were measured at theoutlet of the reaction tube by means of an infrared analyzer. Thetemperature as measured at the inlet of the catalyst-containing reactiontube when CO₂ concentration reached 400 ppm (i.e., electric furnacecontrol temperature) was regarded as Tig.

Gas composition: O₂: 10%, H₂O: 10%, N₂: balance

Flow rate: 25 L/min

Temperature elevation rate: 10 degrees (° C.)/min

Table 1 shows the thus-measured Tig corresponding to the respectivecatalyst-coated particulate filters produced in Examples 1 to 6 andComparative Example 1, as well as the compositions of the catalysts.

TABLE 1 Composition of catalyst Tig Example 1 Ag +Mg/CeO₂(22)ZrO₂(72)La₂O₃(2)Nd₂O₃(4) 403° C. Example 2 Ag +Ca/CeO₂(22)ZrO₂(72)La₂O₃(2)Nd₂O₃(4) 409° C. Example 3 Ag +Ba/CeO₂(22)ZrO₂(72)La₂O₃(2)Nd₂O₃(4) 405° C. Example 4 Ag +Sr/CeO₂(22)ZrO₂(72)La₂O₃(2)Nd₂O₃(4) 417° C. Example 5 Ag + Ba/ZrO₂ 415°C. Example 6 Ag + Ba/CeO₂(30)ZrO₂(70) 407° C. ComparativeAg/CeO₂(22)ZrO₂(72)La₂O₃(2)Nd₂O₃(4) 452° C. Example 1

<Evaluation of Heat Resistance of Catalyst-Coated Particulate Filter>

For evaluation of heat resistance of a catalyst-coated particulatefilter, the balance point temperature of the filter was measured by useof actual exhaust gas. Each of the catalyst-coated particulate filtersproduced in Example 7 and Comparative Example 2, or each of thecatalyst-coated particulate filters produced in Example 7 andComparative Example 2 and then subjected to thermal treatment at 700° C.or 800° C. for 20 hours was placed in a stainless steel holder, and theholder was fixed in a quartz reaction tube. While a portion of exhaustgas discharged from a diesel generator engine (engine displacement: 0.2L) (rotation speed: 3,000 rpm) was distributed to the quartz reactiontube at a flow rate of 30.8 L/min, the quartz reaction tube was heatedfrom outside by means an electric furnace. After the temperature hadreached 300° C., the quartz reaction tube was heated in a stepwisemanner at 20 degrees (° C.)/10 min. The difference in pressure betweenthe inlet and the outlet of the reaction tube containing the particulatefilter was measured, and the temperature at which the pressuredifference is zero was determined. The thus-determined temperature wasregarded as balance point temperature. Table 2 shows the balance pointtemperatures of the respective catalyst-coated particulate filters. Asis clear from data shown in Table 2, the catalyst-coated particulatefilter of Example 7 (i.e., the particulate filter of the presentinvention) exhibits excellent heat resistance (i.e., an increase inbalance point temperature is suppressed even after thermal treatment ata high temperature), as compared with the case of the catalyst-coatedparticulate filter of Comparative Example 2.

TABLE 2 Thermal treatment Type of catalyst-coated Balance pointtemperature, time particulate filter temperature None Example 7 454° C.Comparative Example 2 454° C. 700° C., 20 hours Example 7 455° C.Comparative Example 2 488° C. 800° C., 20 hours Example 7 476° C.Comparative Example 2 502° C.

1. A particulate combustion catalyst characterized by comprising acarrier formed of zirconium oxide; an oxide of at least one metalselected from the group consisting of Ba, Ca, Mg, and Sr, the metaloxide being supported on the carrier in an amount, as reduced to metal,of 0.5 to 30 mass % with respect to the carrier; and metallic Ag or Agoxide, which serves as a catalyst component and is supported on thecarrier.
 2. A particulate combustion catalyst characterized bycomprising a carrier formed of a zirconium-cerium composite oxide; anoxide of at least one metal selected from the group consisting of Ba,Ca, Mg, and Sr, the metal oxide being supported on the carrier in anamount, as reduced to metal, of 0.5 to 30 mass % with respect to thecarrier; and metallic Ag or Ag oxide, which serves as a catalystcomponent and is supported on the carrier.
 3. A particulate combustioncatalyst characterized by comprising a carrier formed of a compositeoxide containing zirconium, cerium, and at least one metal selected fromamong Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr; an oxide of at leastone metal selected from the group consisting of Ba, Ca, Mg, and Sr, themetal oxide being supported on the carrier in an amount, as reduced tometal, of 0.5 to 30 mass % with respect to the carrier; and metallic Agor Ag oxide, which serves as a catalyst component and is supported onthe carrier.
 4. A particulate combustion catalyst according to claim 1,wherein the amount, as reduced to metal, of metallic Ag or Ag oxide,which serves as a catalyst component and is supported on the carrier, is0.1 to 25 mass % on the basis of the total mass of the carrier and anoxide of at least one metal selected from the group consisting of Ba,Ca, Mg, and Sr.
 5. A particulate combustion catalyst according to claim1, wherein the surface of the carrier is provided with SiO₂, TiO₂, ZrO₂,or Al₂O₃, which serves as a binder component.
 6. A particulate filtercharacterized by being coated with a particulate combustion catalyst asrecited in claim
 1. 7. An exhaust gas cleaning apparatus characterizedby comprising a particulate filter coated with a particulate combustioncatalyst as recited in claim
 1. 8. A particulate combustion catalystaccording to claim 2, wherein the amount, as reduced to metal, ofmetallic Ag or Ag oxide, which serves as a catalyst component and issupported on the carrier, is 0.1 to 25 mass % on the basis of the totalmass of the carrier and an oxide of at least one metal selected from thegroup consisting of Ba, Ca, Mg, and Sr.
 9. A particulate combustioncatalyst according to claim 3, wherein the amount, as reduced to metal,of metallic Ag or Ag oxide, which serves as a catalyst component and issupported on the carrier, is 0.1 to 25 mass % on the basis of the totalmass of the carrier and an oxide of at least one metal selected from thegroup consisting of Ba, Ca, Mg, and Sr.
 10. A particulate combustioncatalyst according to claim 2, wherein the surface of the carrier isprovided with SiO₂, TiO₂, ZrO₂, or Al₂O₂, which serves as a bindercomponent.
 11. A particulate combustion catalyst according to claim 3,wherein the surface of the carrier is provided with SiO₂, TiO₂, ZrO₂, orAl₂O₂, which serves as a binder component.
 12. A particulate combustioncatalyst according to claim 4, wherein the surface of the carrier isprovided with SiO₂, TiO₂, ZrO₂, or Al₂O₂, which serves as a bindercomponent.
 13. A particulate combustion catalyst according to claim 8,wherein the surface of the carrier is provided with SiO₂, TiO₂, ZrO₂, orAl₂O₂, which serves as a binder component.
 14. A particulate combustioncatalyst according to claim 9, wherein the surface of the carrier isprovided with SiO₂, TiO₂, ZrO₂, or Al₂O₂, which serves as a bindercomponent.
 15. A particulate filter characterized by being coated with aparticulate combustion catalyst as recited in claim
 2. 16. A particulatefilter characterized by being coated with a particulate combustioncatalyst as recited in claim
 3. 17. An exhaust gas cleaning apparatuscharacterized by comprising a particulate filter coated with aparticulate combustion catalyst as recited in claim
 2. 18. An exhaustgas cleaning apparatus characterized by comprising a particulate filtercoated with a particulate combustion catalyst as recited in claim 3.